Method for the comminution of particulate solid materials

ABSTRACT

A process and apparatus for grinding particulate material to micron or fractional micron size. A high velocity stream of high pressure grinding fluid is passed through a multistage Venturi system into which the material is fed to result in self-attrition of particles by impact. Fluid and entrained material are injected into a mill chamber wherein particles of different sizes are separated and classified. Mill preferably shaped to provide an endless path. Patterns of hydrodynamic flow and vortex motion are produced in the chamber so that a solid particle makes many collisions, these conditions being effected by dispositions of high pressure fluid nozzles, design of solid boundary surfaces and regulation of pressure and temperature of oncoming high velocity fluid. Provision is made for recycling the oversized particles either to the aforementioned multistage Venturi system or to a similar but separate system for regrinding and reintroduction into the chamber.

United States Patent [72] Inventor George E. K. Blythe 37 Ashlawn Road, Hillmorton, Rugby, Warwickshire, England i [21] Appl. No. 832,779 i [22] Filed June 12, 1969 [45] Patented Oct. 19, 1971 [32] Priority June 19, 1968 [3 3] Great Britain [54] METHOD FOR THE COMMINUTION 0F PARTICULATE SOLID MATERIALS 5 Claims, 14 Drawing Figs.

[52] US. Cl 241/5 [51] Int. Cl B02c 19/06 [50] Field of Search 241/5, 39, 43

[56] References Cited UNITED STATES PATENTS 2,846,150 8/1958 Work 241/5 8 3,467,317 9/1969 Stephanoff 241/5 3,491,953 1/1970 Stephanoff 241/39 Prir' nary Exarniner-Othell M. Simpson Attorney-Larson, Taylor & Hinds ABSTRACT: A process and apparatus for grinding particulate material to micron or fractional micron size. A high velocity stream of high pressure grinding fluid is passed through a multistage Venturi system into which the material is fed to result in self-attrition of particles by impact. Fluid and entrained material are injected into a mill chamber wherein particles of different sizes are separated and classified. Mill preferably shaped to provide an endless path. Patterns of hydrodynamic flow and vortex motion are produced in the chamber so that a solid particle makes many collisions, these conditions being effected by dispositions of high pressure fluid nozzles, design of solid boundary surfaces and regulation of pressure and temperature of oncoming high velocity fluid.

, lfrovisionis made for recycling the oversized particles efl'her to the aforementioned multistage Venturi system or to a similar but separate system for regrinding and reintroduction into the chamber.

PAIENTEUDET 19 Ian SHEET 3 OF 7 Pmmtuw 19 m I sum sor 7 METHOD FOR THE COMMlNUTllON F PARTICULATE SOLE) MATERIALS This invention appertains to the comminution of solid materials which are either initially in particulate or analogous fonn or have, prior to such comminution, been reduced to particulate form from comparatively large pieces, rocks or the like.

The general purpose of the process and the apparatus provided by the invention is so to modify the condition of materials as to facilitate handling, packaging, dispensing or use, or/and to enhance the properties or quality of the same.

ln the industrial field, materials comminuted by the method and apparatus of the invention may include pharmaceuticals, including insecticides such as D.D.T., cosmetics, plastics, solid lubricants, pigments for paints and metal-coating, and metal powder for powder metallurgy.

Another group of materials capable of being so dealt with comprises combustible fuels such as coal and colloidal fuel, wood flour, husks and the like.

As to soil nutrients, rock, urea formaldehyde and ash may with advantage be comminuted to a pulverulent form by the method and apparatus of this invention.

Mechanical grinding processes are generally inefficientconsidered from the point of view of energy expended versus surface energy of new surface produced. In mechanical grinding much energy is wasted in friction of the moving parts. Another difficulty with mechanical grinding arises when a very fine product is required. This cannot be obtained merely by leaving the same batch of material for a prolonged period in a mechanical mill. Experience shows that the size distribu tion of particles quickly settles down to an equilibrium one, wherein the breakup of large particles is balanced by the joining together of small ones. Once this stage is reached further expenditure of energy is useless. Thus, to obtain particles of minimum size from a mechanical mill it is practically essential to provide some means of continuously removing the smaller particles.

An object of the present invention is to provide a generally improved and efficient process of comminuting particulate material to a pulverulent form of micron or fractional micron size, as will be hereinafter described.

Another object of the invention is to out this process, a novel form of mechanical moving parts.

The process according to this invention consists in generating a high velocity stream of high pressure fluid, raising the said velocity to a peak value and thereon permitting expansion of the fluid and consequent conversion to kinetic energy of the potential energy of compression thereof in a multistage Venturi system wherein the fluid is increased in velocity and permitted to expand a plurality of times, introducing particulate material transversely into the high velocity stream of fluid at the point where the latter is-permitted to expand for the first time, the material and the fluid being thereby so thoroughly intermixed as to result in self-attrition of particles by impact, injecting the mixture of fluid and entrained particulate material into a chamber, constraining the said mixture to follow a curved path within the chamber so as toeffect separation and classification of particles of different sizes within the mixture following said path, diverting from the chamber the outermost portion of the mixture following said path thereby removing particles above a certain predetermined range of sizes from the chamber, introducing said removed portion transversely into either the said high velocity stream of fluid flowing through the multistage Venturi system or into a similar stream flowing through a similar system at the point where the fluid is permitted to expand for the first time, the material contained in the said removed portion and the fluid being thereby so thoroughly intermixed as to result in further self-attrition of the removed particles by impact, and injecting the mixture of fluid and entrained particulate material into the curved path in the chamber for further separation and classification, particles beneath the said predetermined range of sizes being continuprovide, for carrying grinding mill having no ously and automatically removed from the chamber throughout the process.

The grinding fluid employed may be compressed air or, for oxidizable or combustible materials, any appropriate inert gas under pressure. Alternatively, the said fluid may consist of superheated steam.

By a multistage Venturi system is meant one having two or more stages.

The size reduction of particles may take place wholly or principally in the Venturi system. It is, however, also possible for the particles to be only preliminarily rough ground in the Venturi system and finely ground within the mill chamber. ln the both cases the oversized particles may advantageously be recycled either to the Venturi grinding zone or to an entirely separate multistage Venturi system for regrinding.

Since the density of the solid material to be ground is several thousand times that of the grinding fluid, a fluid velocity of the sonic to supersonic order of about 1000 feet/second is usually necessary in order that the rate of supply of the raw material into the grinding zone or zones may be of a reasonable order of magnitude and yet satisfy the condition that in the interests of efficiency the mass of raw material supplied per unit time and the mass rate of supply of the grinding fluid are to be comparable. Thus, a sonic to supersonic flow of fluid is typical of what is meant by the expression a high velocity.

Such high velocities require that the rate of supply of material into the grinding zone or zones shall be maintained at a high and substantially uniform level; otherwise there will be short periods of time during which the fluid stream is overloaded and the velocity of the combined stream (when the relative velocity of fluid and material has fallen to zero, that is to say the velocity imparted to the material particles) is consequently small, while for the remaining periods the stream is running light and although the material particles attain high velocities the mass of material supplied per unit time is much smaller than it need be.

Comminution, classification and collection of particulate material in a grinding mill is achieved by producing in various parts of the mill various patterns of hydrodynamic flow, vortex motion, turbulence and possible shock-fronts, so that a solid particle makes a large number of collisions. These conditions are effected in the mill by suitable dispositions of nozzles, appropriate design of solid boundary surfaces and regulation of the pressure and temperature of the incoming high velocity fluid, the particles having had imparted to them as high an energy of collision as possible consistent with their remaining frangible.

Conveniently, there may be brought about, by asecondary circulation within a vortex constituting a classification region, an automatic classification of the solid particles present according to their sizesparticles below and above a certain range of sizes being guided respectively to output and back to a grinding zone.

The particles and grinding fluid leaving the classification region may be directed along paths which produce further size classification by the effects of hydrodynamic forces and gravity.

it will accordingly be appreciated that particles of different sizes may be distributed starting sharply at a predetermined maximum size, with a continuous distribution of smaller sizes.

An important aspect of the process of this invention is that recycling is used to quickly transfer large solid particles from certain parts of the mill to regions where they are subjected to further grinding.

To ensure that the material is fed into the high velocity stream of grinding fluid also at a high velocity, a particulate material injector of the special form hereinafter to be described may advantageously be employed, such an injector functioning not only to effect intense mixing of the material and the fluid but also to produce part of the grinding effect by causing collisions of the solid particles.

That is to say, the material injector provides a mixing chamber for the grinding fluid and the infed particulate solid material, the high velocity conveying stream of fluid being so thoroughly intermixed with the said material as to result in self-attrition of particles by impact and a consequent reduction of particle size.

The particulate material injector of the special form mentioned consists, of a Venturi injector adapted to be fitted to a mill chamber and comprising at least (i) an input jet through which a high velocity stream of grinding fluid in the form of pressurized air or other gas or superheated steam can flow and having adjacent thereto an inlet for feeding into the injector the particulate material to be ground, the interior of the jet first tapering to a comparatively narrow throat designed to raise to a peak value the velocity of the grinding fluid flowing therethrough and then flaring outwardly to a substantially larger diameter to permit of expansion of the fluid and consequent conversion to kinetic energy of the potential energy of compression thereof and (ii) a coaxial Venturi tube or nozzle having a tapering portion into which the input jet leads and a flared portion connected or for connection to or with the chamber.

The design of a grinding mill in which to carry out the improved process of comminution must, to ensure maximum efficiency, be logically based on established scientific laws relating to the motion of a solid particle in a stream of fluid. The scientific principles exploited will be apparent from the further description.

The mill chamber may advantageously be shaped to provide an endless path and is fitted with a plurality of subsidiary compressed air (or other gas) or superheated jets or nozzles. Such a mill is aptly termed a jet energy mill, the subsidiary fluid jets or nozzles extending tangentially or substantially so with respect to the endless flow path and functioning to generate within the mill chamber a fluid vortex motion rather than a linear stream.

In order that the invention may be more clearly understood and readily carried into practical effect, specific examples of jet energy mills in which the process of this invention can be carried out, a typical flow diagram and certain graphs illustrating the economics of the invention will now be described with reference to the accompanying drawings, wherein,

FIG. 1 is a vertical sectional view of a two-stage Venturi product injector combined with a mill chamber of shallow cylindrical or pan form, hereinafter to be described,

FIG. 2 is a plan view of the apparatus shown in FIG. 1,

FIG. 3 is a purely diagrammatic plan view representing the main large vortex and the smaller vortices formed in the mill chamber illustrated in FIGS. 1 and 2,

FIG. 4 is a diagrammatic elevational view of the same chamber depicting a secondary circulation designed to bring about automatic size-classification of solid particles,

FIG. 5 is an elevational view of one complete apparatus including a jet energy mill chamber in the form of a vertically disposed race-track" torus,

FIG. 6 is a diagram, partly in section, illustrating one way in which oversized particles can be scooped out from the mill chamber and fed back into the two-stage input Venturi injector for further grinding therein,

FIG. 7 is a diagram illustrating the manner in which double Venturi product injectors can be arranged in series in conjunction with centrifugal classifiers also in series,

FIG. 8 is a vertical sectional view illustrating an apparatus for collection by cyclone of very fine solid particles in a twostage process,

FIG. 9 is a detail cross-sectional view taken on the line IX- IX of FIG. 8,

FIG. 10 is a vertical sectional view of a particle size-classification mechanism adapted to function on the elutriation principle hereinafter to be defined,

FIG. 11 is a flow diagram schematically outlining a typical cycle of operations within the broad process of this invention, and

FIGS. 12, 13 and 14 are graphs illustrating the economics of the invention in relation to the grinding of coal to micron size particles and to the burning of such micron sized coal with finite excess air, e.g. for generating steam,

Referring to FIGS. 1 and 2, it will be seen that a Venturi product injector is indicated generally at I, this being combined with a mill chamber MC of shallow cylindrical or pan form.

The injector I comprises, in combination, a Venturi inputjet 1 and a coaxial Venturi combining tube or nozzle 2. A high velocity stream of grinding fluid in the form of pressurized air or other gas or superheated steam flows into the injector I, via an inlet tube 3, and from thence through the input jet 1. The interior of this jet first tapers at 1a to a comparatively narrow throat lb designed to raise to a peak value the velocity of the grinding fluid flowing therethrough and then flares outwardly at 10 to a substantially larger diameter to permit of expansion and consequent conversion to kinetic energy of the potential energy of compression of the fluid. The efficient expansion portion of the inputjet 1 leads into the combining tube or nozzle 2 and since the latter also first tapers at 2a to a comparatively narrow throat 2b and then flares outwardly at 2c there is provided a twostage Venturi injector.

The particulate material to be ground to micron or fractional micron size is fed laterally and tangentially into the high velocity fluid stream through a tangential inlet 4 in a hollow cylindrical body 5 within which the input jet 1 is accommodated. This inlet 4 may, as shown, have secured thereto a suitably fluted adapter 6 (see FIG. 2) on to which can be fitted a feed tube or pipe for the material. The arrangement is such that the said material is entrained in the high velocity fluid stream emerging from the jet 1 at a location where the fluid is being permitted to expand after having passed through the narrow throat lb of the jet. The velocity of the fluid and the entrained particulate material is increased as the fluid and the particles therein pass through the narrow throat 2b of the combining tube or nozzle 2 wherein intense mixing and grinding takes place to effect size reduction of particles. The particular injector I illustrated includes a cap 7 for the hollow cylindrical body 5 in the center of which cap is fitted the inlet tube 3 for the grinding fluid which latter flows both through the jet 1 and the coaxial combining tube or noule 2. The appropriate end of the said body 5 may be externally screwthreaded as at 5a to receive a correspondingly internally threaded flange 8 by means of which the injector can be supported or fixed in position. The selection of the material of which the input jet I is made is important since it must not be easily abraded.

Two different modes of operation of the grinding mill are possible. In the first, the main size reduction of particles takes place within the material injectorl itself, the remainder of the mill performing mainly the function ofa particle size classifier and including provision such as that hereinafter to be described, for recycling the oversized particles to the injector. In the second mode, true of the process carried out in the mill illustrated in FIGS. l4, the material injector I merely functions as a preliminary rough grinder, the remainder of the mill, i.e. the mill chamber MC in the illustrated case, performing both the functions of fine grinding and size classification in one continuous operation without recycling to the injector. In fact, the mill chamber MC of shallow cylindrical or pan form is a grinding zone of a jet energy mill as hereinbefore defined inasmuch as it provides a circular endless path and is formed with a plurality of subsidiary compressed air (or other gas) jet orifices 9. The latter extend tangentially or substantially so with respect to the circular endless flow path and function to generate within the mill chamber MC smaller vortices 10 which are near the chamber walls and on the outside ofa large main vortex 11 in the central region of the chamber (see FIG. 3).

Referring to FIG. 1, it will be seen that there is provided upon the peripheral margin of the upwardly dished top 12 of the mill chamber MC an annular feed manifold 13 into which feeds the stream of intermixed high velocity grinding fluid and preliminarily ground particles flowing out of the outwardly flared portion 20 of the combining tube or nozzle 2. This stream flows into the feed manifold 13, via a funnel I4, and from thence into the mill chamber MC through feed inlets formed through the chamber top 12. Completely surrounding the said chamber is a gaseous fluid pressure manifold, i.e. annular pressure belt, 16 into which compressed air or other gas flows through an inlet 17 and from thence into the chamber through the tangentially extending jet orifices 9. A gaseous fluid outlet 18 extends coaxially from the chamber top 12. A concentric cyclone collector 19 collects the ground particles which are deposited in a bin 20.

In any event a suitably arranged outlet (not shown) connects the periphery of the mill chamber MC with the injector l.

The simple jet orifices 9 may, if desired, be replaced by Venturi jets. The axes of such jet orifices or Venturi jets (of which any appropriate number may be provided) are parallel to the bottom of the shallow cylindrical chamber MC and inclined nearly tangentially with respect to the circular section of the cylinder. The fluid in close contact with the chamber walls is at rest. A large vortex 11 is formed in the central region of the chamber and smaller vortices 10 near the walls (FIG. 3). The small vortices ensure that a solid particle will make many collisions with its fellows. In the main vortex ll, other velocity gradients are imposed by two geometrical facts: that the path of a particle of fluid near the wall is longer than that of one near the axis, and that, in order to leave the chamber, any solid or fluid particle must spiral inwards to a central outlet. These velocity gradients are associated with pressure gradients. Because the intensity of the vortex is greater in a plane A-A containing the jets near the bottom of the chamber MC, the pressure difference between the center and the edge of the main vortex 11 is more than the corresponding pressure drop in a parallel plane B-B near to the top of the chamber. The difference between these two pressure drops is used up in maintaining a secondary circulation indicated by the arrows in FIG. 4 and this flow has the effect of accelerating particles of solid or fluid from the periphery towards the central outlet. The secondary circulation continually carries solid particles back to the grinding zones near the jet orifices 9 or Venturi jets, i.e. near to the walls where speeds are high and turbulence is at a maximum. The fact that the secondary circulation is inward near the top of the chamber provides a size-classification mechanism. This is because the inward force acting on a particular solid particle in this region as a result of the action of the flow thereon is related to the area which it presents to the flow at any instant. This force varies from the first power, the square of the radius of a disc or a sphere which would present the same area to the flow at that instant. There is a slow change from the first power law (viscous forces) for the very small particles to second power law (Newton-type force) as the size of the particle increases. Whichever law of force is involved, it acts in competition with the centrifugal force due to the fact that a typical solid particle is moving in a circular path as a result of being carried round by the main flow in the chamber which, as aforesaid, is a large central vortex ll. Evidently thiscentrifugal force is proportional to the mass of the particle, that is to its effective radius cubed. The distance from the centerline at which the inward hydrodynamical and the outward centrifugal forces balance is therefore related to the main cross section of a particular particle, that is to say to its effective radius. Detailed consideration of the hydrodynamics then show that there is obtained a classification of particles according to size, the particles of smaller radius tending to perform orbits of smaller radius (and therefore nearer the centerline of the chamber). By proper design of the outlet (including usually some standup into the chamber) it can be secured that, because of this size-classification mechanism, a particle is very unlikely to reach the outlet unless and until the effective radius of the particle falls below a certain critical value. The

fluid vortex generated within the mill chamber can thus be divided roughly into three parts, viz an outermost part constituting a grinding or reduction zone, an inner part being a withdrawal zone and an intermediate part constituting the classification zone.

Advantages of the turbulence described are that the fluid and solid material are maintained in a well mixed condition, and that the fluid velocity changes rapidly with time and place so that a particle is likely to collide frequently with other particles of different velocities. There is also produced a stirring effect assisting circulation of the particles being ground. Moreover, the main vortex ll acts as a flywheel" to smooth out irregularities arising from nonuniform feeding of the particulate material into the grinding or reduction zone.

Due to the centrifugal acceleration caused by the particles being carried round by the vortex l 1, they will concentrate adjacent to the outer walls where there is turbulence which maintains the particles in suspension in a thin fluid layer at the boundary. This layer of fluid, together with its load of solid particles, is conveyed by the secondary circulation indicated by the arrows in FIG. 4 until it reaches a location at which the radial velocity is inward. This inward velocity carries the smaller particles with it, while the oversize particles are left behind at the periphery to undergo further grinding. Thus, only the smaller particles can reach the withdrawal zone.

But only a proportion of the material particles will be ground to the desired micron or fractional micron size, and some of the oversized material may be carried away from the grinding zone by the motion of the fluid.

The seriousness of this depends on the hardness of the material and the particular application. Accordingly, an important feature of the mill is that not only does the above-mentioned size classification mechanism prevent the oversized particles from reaching the output, but the existence of the secondary circulation (FIG. 4) ensures that these oversized particles are automatically carried back to the grinding zones. Thus, a typical particle may have to pass many times through the grinding zones before it attains the desired size, and it may also be subjected several times to the recycling process described above.

The above described principles also work if the mill chamber is constituted by a pair of coaxial cylinders, the annular gap between these cylinders defining the chamber, collection of the product taking place near the inner of the cylinders. Similarly, a mill chamber in the shape of a hollow torus is satisfactory, collection taking place at the inner side of such torus.

Yet another possible variation is a race-track shape torus, i.e. two half-toruses connected by straight portions. The latter design has the advantage that it can be fabricated and joined up as lengths of pipe.

In short, the geometrical form of a jet energy mill chamber may vary widely and may include even a serpentine form. Whilst the simplest form is probably a shallow cylindrical chamber or pan, such as that shown in FIGS. 1 and 2, other geometries are about as efficient and sometimes easier to make. For instance, the toms or race-track" can be of circular section, fabricated of pipe, or of square or trapezoidal section fabricated of sheet.

One complete apparatus embodying the invention is illustrated, merely by way of example, in FIG. 5. This apparatus comprises, in combination, a jet energy mill in the form of a vertically disposed race-track torus: a two-stage Venturi product injector l of the form hereinbefore described with reference to FIG. l, the direction of the high velocity grinding fluid flowing into this injector and thus into RT being indicated by the downward arrow; a feed funnel 21 for particulate material; a Venturi feed injector 22 at the bottom of said funnel; an infeed pipe 23 extending from the feed injector 22 into the bottom half-torus 24; tangential nozzles or jets 25 fitted to the bottom half-torus; a high velocity fluid manifold 26 having connections 27 with the two-stage Venturi injector I, the feed injector 22 and the tangential nozzles or jets 25; a

product outlet 28 extending laterally from the top half-torus 29 (constituting a recycling bend); and a cyclone collector 30 fitted to the outer end of the said product outlet. At 31 and 32 are shown straight portions of tubing connecting the two halftoruses 24 and 19.

With regard to the complete apparatus just described it is to be very clearly understood that if, as may be, the multistage Venturi injector I is to function as a preliminary grinder for particulate material being introduced into the race-track" torus RT, then there would be provided an inlet for introducing such material into the hollow body 5 of the said injector. In such circumstances, the Venturi type feed injector 22 could be dispensed with and the injector I wholly relied on for feeding particulate material into the mill. Or the injector could still be used as a further means of introducing particulate material into RT. That is to say, the injector 22 in this particular apparatus is optional.

Now whatever the form of a grinding chamber in any appropriate apparatus according to this invention there is always a range of effective solid particle sizes which is held in the said chamber for some time, neither being small enough to escape via the outlet, nor making enough collisions to be quickly reduced to a required size. This effect delays the grinding and so reduces output. Such larger and heavier particles are more likely to be found nearer the outer walls of the grinding chamber than near its center, and the apparatus is designed to scoop such particles out of the chamber and feed them back into a multistage Venturi injector for some size reduction by regrinding. This increases efficiency by speeding up the grinding. Thus, for example, it is possible to scoop out larger and heavier solid particles from the upper portion of the recycling bend constituted by the top half-torus 29 in FIG. 5 and to feed these particles, in the direction of the rightward pointing arrow in this figure back into the two-stage Venturi injector I.

This scooping out technique is diagrammatically illustrated in FIG. 6 as applied to a recycling tubular bend 33 ofa jet energy mill chamber of toms form. In this figure, the outer and inner wall portions of the semicircular bend 33 are indicated by the numerals 33a and 33b respectively. A two-stage Venturi injector I generally of the form described with reference to FIG. I is fitted on the torus-with the combining tube or nozzle 2 extending down thereinto. Extending slidably into and chordally across the outer portion of bend 33 is a scoop member 34. Widthwise, i.e. in a direction at right angles to its length, the member 34 extends right across the section of the bend 33. The inner leading end 34a of the scoop member 34 faces the upwardly swept oncoming intermixed stream of high velocity gaseous fluid and solid particles. The scoop member 34 is longitudinally adjustable inwardly and outwardly (as indicated by the chain lines) by any appropriate mechanism to vary the size of the opening between the outer wall portion 33a and the leading end 34a of the member. In this way, the said member functions as a divider to scoop out" the larger or heavier solid particles from the bend, allowing the smaller and lighter particles to pass around the bend undisturbed. The size of the opening 0 determines which size or sizes of solid particles within the range concerned shall be scooped out. The scooped-out particles are drawn into a pipe 35 through which they are fed tangentially into the hollow cylindrical body of the two-stage Venturi injector I. The said oversized particles are accordingly reduced by regrinding in the injector I and thereupon immediately reintroduced into the torus through the combining tube or nozzle 2. In FIG. 6 the paths taken by the heavier and the lighter solid particles are indicated by arrows. It is most important to realize that the two-stage Venturi injector I may be either one through which the particulate material is originally introduced into the mill chamber, or one which is entirely separate from the material feed and is wholly reserved for the recycling process. In this regard, the injector I shown in FIG. 5 may be reserved for recycling only in which instance the Venturi feed injector 22 may be the only means of feeding particulate material into the "race-track torus RT.

Whether or not grinding takes place wholly or principally in the main injector or only partly in the latter and partly, by vortex motion, in the mill chamber fitted with the subsidiary fluid jets or nozzles, a jet energy mill according to this invention may readily combine in one unit the separate operations of grinding, classification of sizes, return of oversize particles to the grinding zone or zones, and collection of the pulverulent product of micron or fractional micron fineness. The idea in this regard is that various portions of the vortex can be made to perform all these functions.

It is possible to provide, in one and the same apparatus, a plurality of superimposed grinding chambers each with its own high velocity fluid inlet but with the ground material outlets of the chambers leading into a common collector.

Moreover, and as shown in FIG. 7, it is within the scope of the invention to provide a plurality of two-stage Venturi particulate material injectors I which are arranged in parallel, i.e. cell formation, for link-up" with centrifugal classifiers (not shown) also arranged in parallel. Thus, in FIG. 7, a supply of compressed air enters, at 36, a compressed air manifold 37, the common particulate material feed pipe being indicated at 38 and the individual material inlets at 39. In the regions of these inlets the particulate material is entrained in the compressed air and, after being ground, either preliminarily or finally, in the Venturi injectors l, passes from the latter, via pipes 40 into a ground material manifold 41 which is linked up with the centrifugal classifiers.

As will be recalled, a cyclone collector is shown at 19 in FIG. I and at 30 in FIG. 5. It is convenient here to mention that the standard cyclone collector is a typical example of the proper design of solid boundaries to produce desirable effects of hydrodynamic flow. Thus, if a mixture of gas and solid particles is fed nearly tangentially into a tube of conical form, the forces on a solid particle are directed towards the point of the cone defined by this tube, thus achieving separation. This is one of the established scientific laws upon which the design of a grinding mill according to this invention is based.

It has been proved that the efficiency of collection by cyclone of very fine particles suspended in a gas can be improved by making it a two-stage process. Thus, the effective radius of the particles can be increased and their effective density lowered by providing an atomized water-spray (to give droplets of suitable size) near the wide end of the conical cyclone. The droplets in such a case are driven towards the point of the cone by the ordinary cyclone forces referred to above, and sweep up the fine solid particles during their travel. Near the point of the cone in this two-stage process the droplets pass through a heated zone provided by a steamjacket, the water evaporating, but the solid particles still being riven towards the point of the cone by the cyclone forces and entering the collector as a dry product. A two-stage collection by cyclone of ground and classified particles of micron or fractional micron fineness is illustrated in FIGS. 8 and 9. At MC in FIG. 8 is depicted a mill chamber of shallow cylindrical or pan form similar to that already described herein. Mounted on this chamber (but omitted from FIG. 8 for simplicity in illustration) is a two-stage Venturi particulate material injector. At 42 is indicated a cyclone collector having a lower end portion 42a of a conical form and an outlet 43 for ground particles (dust). Surrounding the collector 42 is a steam jacket 44 into which steam generated, in a boiler (not shown), is led via inlets 45. Steam from the jacket 44 passes, via a pipe 46 and a valve 47, into an annular steam manifold 48 surrounding the mill chamber MC. From this manifold 48 the steam issues into the mill chamber through jet orifices formed in the wall thereof. With the valve 47 appropriately set steam, instead of passing into the mill chamber MC, can be caused to flow in the direction of the arrow 49 into a steam trap. In the upper end of the cyclone collector 42 is provided a nozzle belt 50 into which water is injected at 51 to form an atomized spray 52 just above the wide end of the cone 42a. Fine solid particles pass from the mill chamber MC into the upper end of the collector 42 via a pipe 53. The steam outlet 54 form the said mill chamber loads into the upper end of a steam condenser 55. The condenser inlet is indicated at 56, and the condensate level 57 is arranged to control the feed to the condenser. The arrowed vertical dotted line 58 in FIG. 8 represents a connection between the boiler and the condenser 55. The .water droplets, as previously mentioned, are driven downwards towards the lower end of the conical portion 42a and sweep up the ground fine solid particles. Moreover, the water of the droplets evaporated by virtue of the collector walls being heated up steam containedjn the steam jacket 44, leaving the fine solid particles in a dry condition as they are driven from the outlet 43 by the cyclone forces.

A particle of just above the aforementioned critical effective radius can, as the result of a collision, be broken into two (or more) fragments which may be very different in size. These fragments are now all below the critical size and are therefore eventually carried out though the outlet. This means that the combination of grinding and classifying mechanism gives a size-number distribution that starts fairly abruptly at the critical effective radius and varies ,slowly and smoothly for lower effective radii. For some purposes, e.g. paints, powder metallurgy, plaster, such a distribution, with nothing above a certain effective radius but with all sizes below that represented, can be described as ideal, but in other applications the very smallest particles would be wasted. This can occur with drugs where the rate of absorption can be critical, or with insecticides, where too small a particle is ineffective whereas too large a parYic le bloclfs the distribution device. In such cases-the sizes mustbe cut" sharply between very narrow limits. This can be accomplished by combining the sizeclassification mechanism, produced by the hydrodynamics alone which, as will be appreciated, cuts the size distribution very sharply at the top limit, with some device that removes effectively all the particles below a certain size. Many standard methods can be exploited for this purpose; thus, not only is it possible to employ natural or mechanically assisted centrifugal classification ashereinbefore described, but also elutriat1on.

different. It is possible, by exploiting this difference, to ensure that the very smallest particles are led to a place where they can be precipitated or bled off, only those above a certain size reaching the final output. That is to say, the elutriation principle can be exploited in a case where the very finest particles are not required but a narrow cut of sizes is specified for removal. The elutriation principle can be carried out without the use of any moving (in contradistinction to merelyadjustable) parts.

A typical flow diagram relating to a complete apparatus for the comminution of particulate solid materials in accordance with this invention is illustrated in FIG. 11. In this figure the letter a indicates raw material; the letter 1; indicates mechanical grinding and sieving; 0 indicates particulate feed material; a a two-stage Venturi grinder and injector; e preliminary rough ground material;ffluid jet energy mill classification; g small particles; h medium particles; ilarge panicles;j an outer takeoff by which large particles are returned to the two-stage Venturi grinder and injector d; k fluid jet energy grinding zone; and [fine particles the larger ones of which are recycled to g by an innertakeoff at m whilst the remainder pass at n into a cyclone collector.

To afford some approximate idea of the order of reduction in particle size which can be attained by the process of this invention, the following are quoted-merely by way ofexample:

i. Tea leaves or other particulate foodstuffs initially of about one-eighth inch maximum dimension can be reduced to particles of from 3-4 microns.

ii. Pigments (for paints) initially ofa size conforming to 100 mesh B.S.S. can be reduced to particles of 0.1 micron.

iii. Fuels crushed to an initial size of 100/200 mesh B.S.S.

can be reduced to particles of from 4-5 microns.

iv. Soil nutrients initially conforming to 270 mesh B.S.S. can

be reduced to a particle size of, say, 0.5 micron. v. The following table shows particle size distributions of thoria before and after treatment in apparatus of this invention:

Class in microns (expressed as number percent falling into each class) 0.6 Run and 0.6 0.8 1 to 1.5 2 3 t0 4.5 9 12 18 Material N 0. Cyclone below to 0.8 to 1 1.5 to 2 to 3 4.5 to 6 to J to 12 to 18 to 25 Thoria as supplied 25. 4 16. 4 10. 1 0. 4 7. 2 9. 5 10. 2 5. 0 4. 2 U. 8 0. 7 0. 2

1 53 25 11 5 3 2 l 0. 1 Thoria after reduction 1 2 58 16 11 7 5 2 1 0.2

1 16 10 8 5. 7 3. l l. 3 0. 7 2 2 16 8. 5 7. 2 4. 4 2. 3 1. 5 0. 1 3 47 21 12 11 6 1. 8 0. J 0. 3

l 53 18 11. 2 9.1 4. 8 2. 9 0. 9 0. 1 3 2 58 17 9 7. 4 4. 5 2. I l. 4 0. 4 3 62 18 8. 3 5. 8 3. 2 1. 5 0. 7 0. 4

stituted by an impact plate 61, and finally travels upwardsagain, encountering shutters 62 the positions of which can be adjusted by turning control shafts 63. The combined results of gravity and hydrodynamics are that the paths taken by small and large solid particles within the device can be made very It has previously been stated that the design of a grinding mill according to the invention must, to ensure maximum efficiency, be logically based on established scientific laws relating to the motion ofa solid particle in a stream of fluid. Most of the laws and principles concerned have already been stated in the context of the foregoing description with reference to drawings. However, two further such statements require to be set out to complete the picture and these are as follows:

I. The preheating of gas or. steam result in higher tip velocities and more efficient grinding by virtue of the higher velocities of the solid particles.

2. The precoolingof gas, and the exploitation of the cooling that occurs on expansion can be adopted to deal with solids like D.D.T., which are sticky and waxy at room temperature, but become frangible if cooled.

The outstanding advantages resulting from exploitation of the stated scientific principles are, in certain specific instances, accompanied by the disadvantage of greater operating costs compared with the cost of driving a mechanical mill. Accordingly, another aim of the present invention lies in providing means whereby power consumption (in whatever form) with particular reference to its use in the final comminution with the fluid jet energy system can be appreciably reduced, thereby eradicating the disadvantage above mentioned. The flow diagram constituting FIG. 11 schematically outlines the cycle of operations which accounts, in part only, for the substantial improvement in operating costs compared with the use of an external supply of energy. By appropriately combining the mechanism of combustion with the results gained from fundamental research into the physical state and behavior of gaseous fluids, the balance of improvements of economic working is achieved.

The body of the specification has already described the advantages resulting from the disposition and use of the various components of the system, as exemplified in the flow diagram (FIG. 11). It now only remains to deal with the other contributory economic facets of working which are as follows:

Micron coal with finite excess air can, as will be appreciated from a consideration of FIGS. 12, 13 and 14 be used for generating steam at thermal efficiency upwards of 92 percent. Additionally, the fullest possible use may be made of the sensible heat remaining in the waste products of combustion for the purpose of preheating the compressed air from atmospheric temperature of 60 F. to 260 F. This results in a most advantageous increase in the pressure or volume of the air used to reduce the coal fed to the fluid energy mills to produce coal particles of one micron, thereby deriving simultaneously the proved benefits resulting from the use of preheated air for direct combustion purposes. Use can also be made of the remaining volume of waste products of combustion to transmit, at normal performance, their sensible heat content to the boiler feed water, such as will ensure under normal working conditions an increase in temperature from atmospheric at 60 F. to 180 F. By adding to these thadvantages of correctly designed nozzles, correctly disposed in the grinding zone or zones, it will be appreciated from the results obtained that there is a high efficiency of operation, with attendant decrease in operating costs.

In FIG. 12, the axis of abscissas (x) is marked to designate the mean diameter of particles in microns, whereas the axis of ordinates (y) is marked to shown the integral percentage by weight-all when grinding coal in a mill. On this particular graph, solid line curves relate to coal extract, the dotted curve to British bituminous coal and the chain line curves to German fine coal. The steep curves at the left-hand side of the graph relate to micronized coals and, for purpose of comparison, the less steep curves relate to conventionally pulverized coals.

The graphs constituting FIGS. 13 and 14 relate to the burning of micronized coal. Thus, FIG. 13 is a graph concerned with minimum burning times at microns. The x axis is marked to show air-coal ratio-V GO (Nm lKg), whereas the y axis is marked in seconds of burning time. FIG. 14, on the other hand, is a graph concerned with maximum burning temperatures at 10 microns, the x axis being designated V GO (Nm /Kg) and the y axis being designated TP(K.).

Comminuting apparatus of this invention may, ifdesired, be incorporated in, or combined with, any other appropriate plant or apparatus for progressively reducing the size of comparatively large pieces, rocks or the like of raw material and thus converting the latter into a particulate form suitable for pulverizing by the improved process of the invention.

This invention includes, as a feature, a comminuted product of micron or fractional micron fineness produced by the herein described process.

I claim:

1. A process of comminuting a particulate material to a pulverulent form of micron or fractional micron size, which comprises generating a high velocity stream of high pressure fluid,

raising the said velocity to a peak value and thereon permittmg expansion of the fluid and consequent conversion of the potential energy of compression thereof to kinetic energy in a multistage venturi system wherein the fluid is increased in velocity and permitted to expand a plurality of times, introducing particulate material transversely into the high velocity stream of fluid at the point where the latter is permitted to expand for the first time, the material and the fluid being thereby so thoroughly intermixed as to result in self attrition of particles by impact, injecting the mixture offluid and entrained particulate material into a chamber, constraining the said mixture to follow a curbed path within the chamber so as to effect separation and classification of particles of different sizes within the mixture following said path, diverting from the chamber the outermost portion of the mixture following said path thereby removing particles above a certain predetermined range of sizes from the chamber, introducing said removed portion transversely into either the said high velocity stream offluid flowing through the multistage venturi system or into a similar stream flowing through a similar system at the point where the fluid is permitted to expand for the first time, the material contained in the said removed portion and the fluid being thereby so thoroughly intermixed as to result in further self attrition of the removed particles by impact, and injecting the mixture of fluid and entrained particulate material into the curved path in the chamber for further separation and classification, particles beneath the said predetermined range of sizes being continuously and automatically removed from the chamber throughout the process.

2. A process according to claim I, wherein the said mixture is constrained to follow a path within the chamber which results in further selfattrition of particles by impact.

3. A process according to claim 2, wherein the said mixture is constrained to flow in a turbulent manner within the chamber thereby to effect further self-attrition of particles by impact.

4. A process according to claim 1, wherein the said mixture is acted upon by a plurality of fluid jets within the chamber, thereby to achieve said path of flow therein.

5. A process according to claim 1, wherein the said mixture is acted upon by boundary surfaces provided within the chamber, thereby to achieve said path of flow therein. 

2. A process according to claim 1, wherein the said mixture is constrained to follow a path within the chamber which results in further self-attrition of particles by impact.
 3. A process according to claim 2, wherein the said mixture is constrained to flow in a turbulent manner within the chamber thereby to effect further self-attrition of particles by impact.
 4. A process according to claim 1, wherein the said mixture is acted upon by a plurality of fluid jets within the chamber, thereby to achieve said path of flow therein.
 5. A process according to claim 1, wherein the said mixture is acted upon by boundary surfaces provided within the chamber, thereby to achieve said path of flow therein. 